In water treatment in recent years, membrane technology is applied to many cases. For example, water treatment in a water treatment plant, microfiltration membranes and ultrafiltration membranes are used, and reverse osmosis membranes are used for seawater desalination. Further, water for semiconductor production, boiler feed water, water for medical service and pure water for laboratory, reverse osmosis membranes and nanofiltration membranes are used. Furthermore, to sewage water/waste water treatment, a membrane-separation activated sludge method using microfiltration membranes and ultrafiltration membranes is applied.
These separation membranes are broadly divided into a plain membrane and a hollow-fiber membrane. Among these membrane, the plain membrane mainly formed from synthesized polymers is often used generally in combination with a support such as a nonwoven fabric or a web since it is inferior in mechanical strength by a membrane having a separation function alone.
Generally, a membrane having a separation function and a support are unified by a method of fixing the membrane and the support to each other by applying a polymer solution, which is a raw material of the membrane having a separation function, onto the support such as a nonwoven fabric, a fabric or the like and spreading the polymer solution. Further, in a semi-permeable membrane such as a reverse osmosis membrane or the like, the membrane and the support are unified by a method in which a supporting layer is formed by applying a polymer solution onto the support such as a nonwoven fabric, a fabric or the like and spreading the polymer solution, and then a semipermeable membrane is formed on the supporting layer.
Therefore, the nonwoven fabric, the fabric and the like to become a support require such an excellent film-forming property that when applying and spreading a polymer solution, the polymer solution does not cause strike through due to excessive permeation, membrane substances do not peel and defects such as nonuniform membranes and pinholes due to fuzz of the support are not produced.
Further, in the case of a semipermeable membrane such as a reverse osmosis membrane often used under a high pressure, particularly, the support requires high mechanical strength and high dimensional stability.
Hitherto, as such a support for a separation membrane, a support for a separation membrane comprising a nonwoven fabric having a multilayered structure, in which a two-layered structure including a surface layer having large opening and surface roughness using thick fibers and a backside layer having small opening and a compact structure using thin fibers is basic, is proposed (Japanese Examined Patent Publication No. 4-21526). Further, in a support for a semipermeable membrane comprising a nonwoven fabric for applying and spreading a polymer solution for forming a semipermeable membrane to form a membrane, a support for a semipermeable membrane, in which the nonwoven fabric is a nonwoven fabric having a two-layered structure formed by laminating and unifying a low-density layer where air permeability is 5 to 50 cc/cm2/sec and a high-density layer where air permeability is 0.1 cc/cm2/sec or more and less than 5 cc/cm2/sec, and overall air permeability is 0.1 cc/cm2/sec to 4.5 cc/cm2/sec, is proposed (Japanese Examined Patent Publication No. 5-35009).
However, since these supports for membrane comprise composed of short fibers instead of a continuous fiber, a nonuniform membranes and defects may be produced due to fuzz. Furthermore, in these documents, there is no proposal about strength of the nonwoven fabric, and since there is not detail description about strength, there is a problem that adequate mechanical strength and dimensional stability cannot be achieved as a support for a membrane.
Further, aside from these supports for a separation membrane, as such a support for a separation membrane, a support for a semipermeable membrane comprising a nonwoven fabric in which an average of a breaking length of elongation of 5% in a machine direction (MD) and a breaking length of elongation of 5% in a crosswise direction (CD) is 4.0 km or more and air permeability is 0.2 to 10.0 cc/cm2·sec is proposed (Japanese Patent Publication No. 3153487). However, this support for a semipermeable membrane is not composed of continuous filaments, but comprises a nonwoven fabric composed of short fibers produced by a papermaking method. Therefore, to obtain a semipermeable membrane having required high mechanical strength, this requires many complicated steps, for example, yarn is stretched in warm water bath after melt spinning, and subsequently double refraction of fiber composing a nonwoven fabric is significantly increased by stretched heat treatment and/or relaxed heat treatment, and heat shrinkage stress is set at a specified range. Accordingly, the support for a semipermeable membrane of this proposal has problems that the support becomes expensive, and since it comprises a nonwoven fabric composed of short fibers, a nonuniform membranes and defects may be produced due to fuzz.
Therefore, it could be helpful to provide a support for a separation membrane, which comprises a long-fiber nonwoven fabric composed of thermoplastic continuous filaments and exhibits an excellent film-forming property and excellent mechanical strength when used for supporting a separation membrane such as a microfiltration membrane, an ultrafiltration membrane, a nanofiltration membrane and a reverse osmotic membrane, and a method for producing the same.
It could also be helpful to provide a separation membrane and a fluid separation element each utilizing the support for a separation membrane.